FIP4F8S datasheet(32/33 Pages) FUJI

32 / 33 page

AN-087E Rev.1.0

April-2011

FA5526/5527/5528/5536/5537/5538

Fuji Electric Co., Ltd.

http://www.fujielectric.co.jp/products/semiconductor/

(Reference)

Sub-harmonic oscillation and slope

compensation

peak-value-control current

mode,

when

the

converter operates in an inductor-current continuous mode

and at duty cycle of 50% or more, the current may oscillate

at an integral multiple of switching frequency. This

oscillation is called subharmonic oscillation.

Fig. 43 shows an example of inductor current waveform

when a subharmonic oscillation occurs.

It is found that ON and OFF periods vary while the peak

value of an inductor current, switching cycle and current

slopes during ON and OFF periods remain unchanged.

The harmonic oscillation may increase ripple voltage

contained in the output voltage or cause an unusual noise.

The subharmonic oscillation can be prevented by giving

a certain gradient to the threshold of the peak current as

shown in Fig. 44. This is called slope compensation.

Generally, the gradient of slope compensation required

for preventing a subharmonic oscillation is given by the

following relational expression:





Here,

Lu：Gradient of an inductor current during the ON period

Ld：Gradient of an inductor current during the OFF period

Kc：Gradient of slope compensation

The above parameters are shown in Fig.45.

Fig.43

Inductor current without slope compensation

Fig.44

Inductor current with slope compensation

Fig.45

Inductor current without slope compensation

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